Selective CO Evolution from Photoreduction of CO2 on a Metal

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Selective CO Evolution from Photoreduction of CO2 on a Metal-Carbide-Based Composite Catalyst Yu-Long Men, Ya You, Yun-Xiang Pan, Hongcai Gao, Yang Xia, Dang-Guo Cheng, Jie Song, Da-Xiang Cui, Nan Wu, Yutao Li, Sen Xin, and John B. Goodenough J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08552 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 18, 2018

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Journal of the American Chemical Society

Selective CO Evolution from Photoreduction of CO2 on a MetalCarbide-Based Composite Catalyst Yu-Long Men,†,# Ya You,‡,# Yun-Xiang Pan,†,* Hongcai Gao,‡ Yang Xia,§ Dang-Guo Cheng,§ Jie Song,† Da-Xiang Cui,† Nan Wu,‡ Yutao Li‡, Sen Xin,‡,* and John B. Goodenough‡,* †

Institute of Nano Biomedicine and Engineering, Department of Instrument Science and Engineering, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China ‡ Department of Mechanical Engineering, The University of Texas at Austin, Austin, TX 78712, USA § College of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou 310027, P. R. China Supporting Information Placeholder ABSTRACT: A selective CO evolution from photoreduction of CO2 in water was achieved on a noble-metal-free, carbidebased composite catalyst, as demonstrated by a CO selectivity of 98.3% among all carbon-containing products and a CO evolution rate of 29.2 µmol h-1, showing superiority to noblemetal-based catalyst. A rapid separation of the photogenerated electron-hole pairs and improved CO2 adsorption on the surface of the carbide component are responsible for the excellent performance of the catalyst. The high CO selectivity is accompanied by a predominant H2 evolution, which is believed to provide a proton-deficient environment around the catalyst to favor the formation of hydrogen-deficient carbon products. The present work provides general insights into the design of a catalyst with a high product selectivity and also the carbon evolution chemistry during a photocatalytic reaction. INTRODUCTION Solar-energy-driven photocatalytic CO2 capture and conversion has attracted great attention, as it utilizes the green solar energy to reduce the CO2 emission and produce valuable carbon-based chemicals.1-6 The photoreduction of CO2 with H2O into syngas (CO and H2) is especially promising, as the syngas serves as an important feedstock in industry, for example, in producing synthetic fuels through the well-established Fischer-Tropsch process. However, the carbon-containing products from the photoreduction of CO2 with H2O is usually a mixture that contains CO, CH4, HCOOH and CH3OH, and the CO selectivity in these carboncontaining products is low. This drawback has hindered the practical realization of photocatalytic production of syngas at an industry scale. Therefore, catalysts with a high selectivity for CO production from photoreduction of CO2 are desired in the field. In pioneering studies, the improved product selectivity during photocatalytic CO2 reduction with H2O has been realized by the use of a noble metal (NM) as co-catalyst, and the selectivity is closely related to the size, dispersion and the exposed crystal surface of the NM nanoparticles as well as its interaction with the support.7-14 Despite the improved selectivity, the high cost and poor manufacturability of the NM catalysts has made them unsuitable for industry-scale applications. In the search of NM-free catalysts with high product selectivity and low cost, transition metal (TM) carbides, e.g. dimolybdenum carbide (Mo2C), are of particular interests owing to their similar catalytic properties to NMs and high activity towards the dissociation of O-H and C=O

bonds, the key steps in the reduction of CO2.15-21 Porosoff et al. studied the CO2 conversion by H2 over various TM carbides at 573 K.20 They observed the highest activity and selectivity towards CO production on Mo2C, as demonstrated by a CO2 conversion rate of 4.67%, a turnover frequency of 66.5, and a CO/CH4 molar ratio of 154.3. However, the study of the TM carbides for the photocatalytic CO2 reduction is still preliminary. Herein, we report a NM-free composite catalyst that selectively converts CO2 into CO at a high efficiency under the irradiation of visible light, which is the main part of sunlight (44%). The composite consists of cadmium sulfide (CdS) nanoparticles supported on Mo2C nanowires; the CdS acts as the visible light absorber and the Mo2C nanowires catalyze the reaction. The catalyst was synthesized through a facile two-step chemistry including solutionbased preparation of Mo2C nanowires and in-situ growth of CdS nanoparticles on the nanowires, as is illustrated in Figure 1a. For simplification purpose, a typical material that is synthesized at a mass ratio of Cd(CH3COO)2 to Mo2C of 1:2 is denoted as CM0.5. RESULTS AND DISCUSSION Material Characterizations. Figure 1b and Figure S1 present the morphological characterization results of the as-prepared Mo2C nanowires, which show that the nanowires have an average length longer than 2 µm and a width of ca. 100 nm. Energy dispersive X-ray (EDX) elemental mappings in the inset of Figure 1b further reveal the uniform distribution of Mo/C on the nanowires. The diffraction peaks on the X-ray powder diffraction (XRD) pattern (blue pattern in Figure 1c) collected from the nanowires are assigned to orthorhombic Mo2C (PDF#77-0720). Referring to the Raman spectrum of the Mo2C nanowires (Figure S2), the peaks at 660 and 818 cm-1 correspond to the Mo-C bond while the peak at 992 cm-1 corresponds to the Mo=C bond.22,23 The D and G bands of carbon are also noted in the Raman spectrum (yet the intensities are significantly lower than those of the carbide peaks), suggesting the presence of trace amount of pyrolytic carbon residues in the prepared Mo2C. The X-ray photoelectron spectroscopy (XPS) Mo 3d spectrum of the Mo2C nanowires are fitted into four peaks at 228.7, 231.8, 232.9 and 235.9 eV (Figure 1d). The peaks at 228.7 and 231.8 eV are assigned to the 3d5/2 of Mo2+ and 3d5/2 of Mo (in the Mo-Mo bond) in Mo2C, and the peaks at 235.9 and 232.9 eV are assigned to the 3d3/2 and 3d5/2 of Mo6+ in MoO3.24,25 The above finding is reasonable as the air exposure of Mo2C may lead to slight oxidation of surface Mo2+, yielding MoO3.25,26 However, no peak that can be assigned to crystalline MoO3 is found in the XRD patterns,

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which indicates the amorphous nature of MoO3. In the XPS spectrum of the carbon species (Figure 1e), which is calibrated by C 1s peak, the peaks at 283.8, 284.6, 285.7, 288.6 eV are assigned separately to the carbide carbon, sp2-hybridized aromatic carbon (C=C bond), C-O bond and C=O bond, which also suggests the co-existence of C residues.24 The Mo2C nanowires have a Brunauer-Emmett-Teller (BET) surface area of 102.1 m2 g-1 according to the N2 adsorption/desorption analysis (Figure S3).

phase) and solubility (in water phase) of CO2 during the reaction process. In this way, one can directly compare the intrinsic catalytic properties and performance of the catalysts. TEOA serves as sacrificial reagents for capturing the photogenerated holes in the CdS, thereby releasing the photogenerated electrons for reaction with CO2 molecules adsorbed on the catalyst. The bare CdS nanoparticles, bare Mo2C nanowires, and CdS/Mo2C composites synthesized with different mass ratios of Cd(CH3COO)2 to Mo2C were employed during the test. The bare Mo2C nanowires and CdS nanoparticles show almost no activity under irradiation (Figure 2b and Table S1). CO accounts for the main carboncontaining product during the photocatalytic CO2 reduction on CM0.5, with an evolution rate of 29.2 µmol h-1 (Figure 2b and Table S1). The CO selectivity, which is defined as the molar percentage of CO among all the carbon-containing products, is 98.3% on CM0.5 (Figure 2b and Table S1). Both the CO evolution and selectivity on CM0.5 are higher than those obtained on the NM-based photocatalysts reported in the literature (Table S2).31-38 Apart from CO, H2 and O2 were also detected as products of the photocatalytic CO2 reduction on CM0.5 with evolution rates of 60.4 and 24.6 µmol h-1, respectively (Table S1). The photocatalytical stability of CM0.5 was tested in five consecutive runs with a time duration of 4 h for each run. After each run, the light irradiation was stopped, the reactor was evacuated and then refilled with CO2 of 1.01 bar (1 atm) yet without washing the photocatalyst or adding fresh aqueous solution. According to Figure 2c, CM0.5 shows excellent stability with a slight decrease of 1.8% in the total amount of CO produced after five runs (from 112.0 µmol in the first run to 110.0 µmol in the fifth run).

Figure 1. (a) Schematic illustration showing the preparation of CdS/Mo2C nanowires. (b) SEM image and elemental mappings of the Mo2C nanowires. (c) XRD patterns of the Mo2C nanowires and CM0.5. (d) Mo 3d and (e) C 1s XPS spectra of the Mo2C nanowires. Figure 2a shows the transmission electron microscopic (TEM) image of the CM0.5, which reveals a uniform distribution of the CdS nanoparticles (~5 nm) throughout the Mo2C nanowire. The lattice fringe that corresponds to a CdS(111) plane with a distance of 0.35 nm can be clearly defined from the high-resolution TEM image (inset of Figure 2a).27,28 The above result agrees with the XRD pattern of the CM0.5 (Figure 1c), on which a new peak appears at 26.5 °, which matches well with the (111) plane of cubic CdS (PDF#75-0581). As compared with the pristine Mo2C nanowires, all the Mo 3d peaks in the XPS spectrum of the CM0.5 show slight shifts towards higher binding energy (Figure S4). The shifts reflect the interaction of Mo2C nanowires with the CdS, possibly via the formation of Mo-S bonds.29,30 The XPS C 1s spectrum of the CM0.5 (Figure S5) does not show any significant variation compared with that of the Mo2C nanowires. The support of CdS nanoparticles also helps to increase further the BET specific surface area of the composite catalyst to 121.4 m2 g-1 (Figure S3). Photocatalytic Reduction of CO2 with H2O. The photocatalytic CO2 reduction experiment was carried out at 25 °C under irradiation of visible light (λ > 420 nm), in a closed gas circulation-evacuation reactor that contains 10 vol% aqueous solution of triethanol amine (TEOA). A far excessive amount of CO2 was introduced in the reactor to minimize the environmental influences on CO2 conversion, such as changes in pressure (in gas

Figure 2. (a) TEM and HRTEM images of CM0.5. (b) CO evolution rate in the photocatalytic CO2 reduction with H2O on CM0.5, CdS/Pt, pure Mo2C nanowires and pure CdS nanoparticles. (c) Stability of CM0.5 and CdS/Pt in the photocatalytic CO2 reduction with H2O. (d) GC-MS spectra (m/z =28, 29) analyses of the carbon source of the evolved CO in the photocatalytic reduction of 13CO2 on CM0.5. Reaction conditions: 100 mL water solution containing 10 vol% TEOA, 100 mg photocatalyst, 300 W Xelamp (>420 nm). A NM-based CdS/Pt photocatalyst was used for comparison. By varying the Pt content, the optimal CO evolution rate and selectivity on CdS/Pt are 5.8 µmol h-1 and 72.5%, respectively (Figure S6 and Table S1). These values are much lower than those


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Journal of the American Chemical Society obtained on CM0.5. In addition, the stability of CdS/Pt is poorer than that of CdS/Mo2C, as demonstrated by a significant decrease of 12.1% in the total amount of CO produced after five runs (Figure 2c). Thus, the NM-free CM0.5 is a competitive alternative to CdS/Pt for the photocatalytic CO2 reduction. To figure out the source of CO production, five control experiments (Ex.) were conducted, including: (i) Ex. with CM0.5, CO2 and H2O, but without light; (ii) Ex. with CO2, H2O and light, but without CM0.5; (iii) Ex. with CO2, H2O, light and CdS, but without Mo2C; (iv) Ex. with CM0.5, light and H2O, but without CO2; and (v) Ex. with CO2, H2O, light and Mo2C, but without CdS. No CO was detected in all the experiments, indicating that CO can only be generated from the photocatalytic CO2 reduction on CM0.5 in the presence of H2O. In addition, an isotopic experiment with 13CO2 as the reactant was performed to determine the source of carbon in the CO product. According to Figures 2d and S7, the gas chromatography-mass spectrometry (GC-MS) spectrum shows a strong m/z peak at 29 (13CO), indicating that CO2 is the major carbon source of the evolved CO. Role of CdS in the Photoreduction. The CdS nanoparticles respond to visible light according to the ultraviolet-visible (UVvis) spectra (Figure S8). This result is in good agreement with previously reported results, which show that the CdS nanoparticles have a high visible light absorbance.27,28,39-41 CM0.5 exhibits a light absorption behavior similar to the pure CdS nanoparticles (Figure S8), indicating that the CdS component in CM0.5 is mainly responsible for light absorption during the photocatalytic process. Hence, the absence of CdS nanoparticles is the reason for the zero CO evolution in the photocatalytic CO2 reduction on the pure Mo2C nanowires (Figure 2b).

Figure 3. (a) PL spectra and (b) photocurrent-time profiles of the pure CdS nanoparticles, CdS/Pt and CM0.5. (c) CO2 adsorption capacities of the pure CdS nanoparticles, the pure Mo2C wires, CdS/Pt and CM0.5. (d) FTIR spectra observed after the CO2 adsorption on the pure CdS nanoparticles, CdS/Pt and CM0.5. Role of Mo2C in the Photoreduction. The role of the Mo2C nanowires in the photocatalytic CO2 reduction is multiple. First, the Mo2C nanowires function as a support that suppresses the aggregation of the CdS nanoparticles. This role is demonstrated by the TEM observation shown in Figure 2a. The uniform dispersion of CdS nanoparticles on Mo2C is essential for a more efficient light absorption.27,28 Second, the Mo2C nanowires enhance the separation of the photogenerated electron-hole pairs formed on the CdS nanoparticles, and promote the electron transfer to catalytically active sites.

The ability of CdS/Mo2C to separate electron-hole pairs was studied by using the photoluminescence (PL) spectra (Figure 3a) and the transient photocurrent response (Figure 3b). An intensive PL peak at about 490 nm is observed for the pure CdS nanoparticles and assigned to the recombination of photogenerated electrons and holes (Figure 3a).27,28,41-44 For CM0.5, the PL peak at 490 nm almost vanishes, implying more efficient electron-hole separation (Figure 3a). Moreover, the CM0.5 shows a significantly higher transient photocurrent response than that of the pure CdS and CdS/Pt (Figure 3b), which also indicates faster electron-hole separation and electron transfer on CM0.5. As more electrons are provided for the reduction reaction, it is reasonable for CM0.5 to show a higher photocatalytic efficiency. In addition, the Mo2C nanowires provide active sites for CO2 adsorption and conversion. Figure 3c shows the CO2 adsorption capacity of the pure CdS nanoparticles, pure Mo2C nanowires, CdS/Pt, and CM0.5. The CO2 adsorption capacity of CM0.5 (1.49 mmol g-1) is slightly higher than that of the pure Mo2C nanowires (1.40 mmol g-1), but significantly higher than that of the pure CdS nanoparticles (0.16 mmol g-1) and CdS/Pt (0.66 mmol g-1). The high adsorption capacity of CM0.5 may be attributed to its large specific surface area, which is mostly contributed by the Mo2C nanowires. Although the actual CO2 adsorption and reaction process occurs in the water phase, it is reasonable for CM0.5 to show a high CO2 adsorption as the nature of CO2 adsorption on the catalyst, i.e., the chemical interaction between the C=O bond of CO2 and the exposed surface atom of the catalyst, remain unchanged. With more CO2 adsorbed on the photocatalyst, more CO2 (reactant) will participate in the forward reaction, which is beneficial for shifting the chemical equilibrium towards the reduction products. The above point is further demonstrated by the Fourier transformation infrared (FTIR) spectra, which show characteristic bands at 1260, 1450 and 1560 cm-1 (Figure 3d). These bands can be assigned to CO2−, which is formed via capture of photogenerated electrons by the adsorbed CO2 and accounts for a key intermediate species in CO2 conversion.45,46 The most intensive CO2− bands are observed on CM0.5 (Figure 3d), which indicates a significantly elevated concentration of electroactive CO2 species on the catalyst surface. We also explored the CO2 adsorption on Mo2C with density functional theory (DFT) calculations with the Mo2C(100) surface as a model. The surface consists of alternatively arranged Mo and C layers and can be either Mo- or C- terminated.47-49 Figure S9 shows the most stable structures of CO2 adsorption on Mo- and C-terminated Mo2C(100) by comparing various sites for CO2 adsorption. On the Mo-terminated Mo2C(100), CO2 is adsorbed via Mo-C and Mo-O bonds with an adsorption energy of 0.78 eV. On the C-terminated Mo2C(100), the CO2 adsorption is realized via C-C and C-O bonds with an adsorption energy of 0.22 eV. The Bader charges of the adsorbed CO2 on Mo- and C-terminated Mo2C(100) are −0.41|e| and −0.16|e|, respectively, both of which indicate electron transfer from Mo2C to CO2. The results are consistent with the FTIR spectra in Figure 3d. The higher adsorption energy and more negative charges for the adsorbed CO2 on Mo-terminated Mo2C(100) indicate that Mo2C with exposed Mo atoms could be energetically more favorable for CO2 adsorption and activation. Thus, we employed the Mo-terminated Mo2C(100) to study further the CO2 conversion reaction on Mo2C.



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also indicates that the CO2 conversion into CO results in formation of an O-Mo-C structure whether the proton is present or not (Figure 4a, b). This result agrees well with the previous work by Chen et al, where the oxycarbide is proposed to be formed during the catalytic conversion of CO2 into CO on Mo2C.20 Reaction Mechanism. Possible reactions occurring during the photocatalytic reduction of CO2 on CM0.5 in the presence of H2O are listed as follows. Light illumination on CdS generates electron-hole pairs (eq. 1). The electrons are separated from the holes and transferred through the Mo2C nanowire to the adsorbed CO2. With a lone pair at the central N atom, TEOA functions as a Lewis base that takes a proton from H2O, yielding a protonated TEOA (TEOAH+, see Figure S10a) and a hydroxide anion according to eq. 2. Similarly, TEOA captures the hole from CdS through eq. 3, forming a positively charged aminyl radical (TEOA+, refer to Figure S10b).50 The as-formed TEOA+ then undergoes a selfdeprotonation process to enable structural rearrangement into a carbon centered radical (TEOA•, see Figure S10c) through eq. 4.50 The radical is highly reductive, so that it is able to capture the hole from the excited CdS (eq. 5) to yield an iminium cation (TEOA⊕, see Figure S10d), which further reacts with water, degrading into glycolaldehyde and diethanol amine and releasing a proton (eq. 6). The degradation of TEOA and generation of protons during the photocatalytic process has been proved by a slightly decreased pH value after the reduction reaction (Table S3). In case the proton release in eqs. 4 and 6 occurs at the surface of Mo2C, the protons may either participate in the photocatalytic reduction of CO2 (to CO) through eq. 7, or combine with photogenerated electrons to trigger hydrogen evolution according to eq. 8. Otherwise, they may react with the hydroxide anions in the alkaline solution to yield water (eq. 9). Water can also be reduced by the photogenerated electrons at the surface of Mo2C, leading to H2 evolution (eq. 10). On the other hand, the oxygen evolution may result either from recombination of hydroxyl radicals (eq. 11), or reduction of hydroxide anions by the hole from the excited CdS (eq. 12). Figure 4. DFT-calculated relative energy profiles for the conversion of CO2 into CO without protons (a) and with protons (b) on the Mo-terminated Mo2C(100) surface. (c) Proposed virtuous circle for the conversion of CO2 into CO by H2O on the Mo2C nanowires. The balls in cerulean, red, grey and white colors denote Mo, O, C and H atoms, respectively.

CdS + hv → h+ + e− TEOA + H2O → TEOAH+ + OH− TEOA + h+ → TEOA+ TEOA+ → TEOA• + H+ TEOA• + h+ → TEOA⊕ ⊕

In the most stable CO2 adsorption structure on Mo-terminated Mo2C(100), one O atom of CO2 bonds with a Mo atom and the other unbonded O atom protrudes into the environment (Figures S9). If the reaction does not involve any proton, the CO2 dissociation on the Mo-terminated Mo2C(100) produces a CO molecule and an O atom that bonds with the exposed Mo atom (Figure 4a). The dissociation reaction is exothermic by 0.32 eV with an energy barrier of 0.68 eV. On the other hand, there are two possible reaction routes for the CO2 conversion (into CO) in the presence of a proton (Figure 4b). In the first route, the proton interacts with the unbonded O atom followed by C-O bond dissociation and formation of a CO molecule and an OH group. In the second route, the proton interacts with the Mo-bonded O atom to initiate the reduction of CO2 and generate the same products. As the reaction proceeding through either route shows higher dissociation energy (0.36 eV for Route I and 0.57 eV for Route II) than the protonfree reaction, it is thermodynamically preferable to have protons participate in the photocatalytic reaction. Also, the reaction proceeding through Route II shows lower energy barriers (0.53 eV and 0.57 eV) and higher dissociation energy than that proceeding through Route I. Hence, Route II is preferable to Route I, both thermodynamically and kinetically. Besides, the DFT calculation

(1) (2) (3) (4) (5) +

TEOA + H2O → HOCH2CHO + (EtOH)2NH + H CO2 + H+ + e− → CO + •OH 2H+ + 2e− → H2 H+ + OH− → H2O 2H2O + 2e−→ H2 + 2OH− 4•OH → 2H2O + O2 4OH−+ 4h+ → O2+ 2H2O

(6) (7) (8) (9) (10) (11) (12)

Previously, an oxycarbide-based mechanism has been proposed for the conversion of CO2 to CO on Mo2C in the presence of H2, which includes two steps: (i) CO2 is reduced to CO on the surface of Mo2C, leaving an O atom that bonds with a Mo atom of Mo2C to form an oxycarbide (O-Mo-C); (ii) the oxycarbide reacts with H2 to form H2O and Mo2C.20 On the basis of the oxycarbide-based mechanism and the DFT calculation results in Figures 4a and 4b, we suggest a closed cycle for the photocatalytic conversion of CO2 into CO on Mo2C nanowires in the presence of H2O (Figure 4c). Step I of the circle involves the adsorption of CO2 on Mo2C and generation of protons from H2O and TEOA. In Steps II and III, CO2 reacts with the electrons and protons to form CO and OH


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Journal of the American Chemical Society radicals. Step IV of the circleis the desorption of CO from Mo2C and the evolutions of H2 and O2. During the conversion, Mo2C first forms an oxycarbide bonding structure (O-Mo-C) and then recovers to the carbide bond (Mo-C) after the attack of an O atom by a proton. Finally, the use of CM0.5 as the catalyst brings a high CO selectivity. To reveal the underlying reasons for this selectivity, one should take a specific look at the reaction process. Eqs. 10 and 13-17 show the main reactions that may occur during the photocatalytic CO2 reduction in the presence of water.8 CO2 + 2H+ + 2e− → HCOOH CO2 + 2H+ + 2e− → CO + H2O CO2 + 4H+ + 4e− → HCHO + H2O CO2 + 6H+ + 6e− → CH3OH + H2O CO2 + 8H+ + 8e− → CH4 + 2H2O

noted that the electrode potential of the O2/OH− couple falls within the energy range that enables the TEOA+/TEOA couple (Figure 5a and Table S4).52,53 Hence, there is a chance for TEOA+ to be reduced (back to TEOA) by OH−, which is beneficial for recycling the sacrificial reagent. However, since the potential of the O2/OH− couple is significantly higher than the VB of CdS (Figure 5a and Table S4), it is preferential for the OH− anion to take the hole from the VB of CdS to initiate O2 evolution (eq. 12).

(13) (14) (15) (16) (17)

Note that eq. 10 is equivalent to eq. 8, the above reactions can be considered as electrode reactions that simply involve pure gas, pure liquid, and protons as the reactants/products. Given that the partial pressures of all gas species are 1 atm, the electrode potentials of the above reactions change with the proton activity in aqueous solution or, in other words, the pH value. By using the Nernst equation, the potentials of the above electrode reactions can be calculated according to the following equation: E = E0 - 0.0592*pH

(18)

where E is the theoretical electrode potential vs. Normal Hydrogen Electrode (NHE) under normal conditions (298 K, 1 atm) and the given pH value; E0 is the electrode potential vs. NHE at pH = 0. Since the oxygen evolution reaction (i.e., the O2/OH− couple according to eq. 12) is pH dependent, the electrode potential can also be calculated using the above equation. The energy bands of the CdS/Mo2C catalyst were analyzed since they may also affect the photocatalytic reactions. According to the density of states (DOS) analysis (Figure S11a), there is a band gap reduction, from 2.48 eV on the pure CdS to 2.39 eV on CM0.5, which is due to a lowered CB (by 0.07 eV) and a raised VB (by 0.02 eV). The result is consistent with the experimental data obtained from the Mott-Schottky plots (Figures S12 and S13), which show an increased CB energy of CdS after it composites with Mo2C. Also, in the UV-vis spectra (Figure S8), the wavelength corresponding to the adsorption edge of CM0.5 is longer than that of the pure CdS. This result indicates an enhanced visible light absorption on CM0.5 that can also be linked to a reduced band gap. Table S4 shows the theoretical potentials (E) for the key redox reactions (eqs. 10, 12, and 13-17) that occur in acidic (pH = 0) and neutral (pH = 7) aqueous solutions and at the experimental pH value (pH ≈ 9.5 according to Table S3).8,51 The data are plotted versus the energy of the valence band (VB) and conduction band (CB) of CdS in Figure 5a. Under neutral condition, the relative energies of the redox couples in eqs. 10 and 13-17 lie below the CB of CdS. Hence, the excited electron in the CB of CdS has a sufficiently high energy for all the reduction reactions. Raising the pH value from 7.0 to 9.5 decreases the reduction potentials of the above reactions, yet all the redox couples, except the HCOOH/CO couple (eq. 13), are still able to maintain lower energies than the bottom of the CB of CdS. Hence, the reduction products in eqs. 10 and 14-17 are expected to be obtained during the photocatalytic process (HCOOH cannot be obtained). It is also

Figure 5. (a) Energy bands of CdS/Mo2C plotted against theoretical potentials for key redox reactions that may occur during the photocatalytic process in a neutral solution (pH = 7) and at the experimental condition (pH ≈ 9.5). (b) Schematic illustration showing the evolution of carbon-containing compounds under a proton-rich environment. (c) Schematic illustration showing the evolution of carbon-containing compounds under a protondeficient environment. Experimentally, the photocatalytic evolution of CO on CM0.5 is strongly preferred over the other carbon-containing products. Here, we would also like to ascribe the selective CO evolution to the proton environment on the catalyst. In our preliminary work, photocatalytic reduction of CO2 into various kinds of carboncontaining products (e.g., CO, CH4, CH3OH, HCHO and HCOOH in a descending order by evolution rate) was reported on a carboncoated, Pt-supported In2O3 nanobelt in an aqueous solution of TEOA.8 The low selectivity of carbon-containing products was accompanied by supressed H2 evolution, which was attributed to a tendentious proton transfer to the absorbed CO2 over proton



recombination to generate H2. Nevertheless, the selective CO production in this work is accompanied by boosted H2 evolution (Table S1). Since H2 evolution is the main competitive process to that of CO2 reduction, it is reasonably believed that the suppressed H2 evolution in the previous work provides more protons to the photocatalytic CO2 reduction, which is beneficial for formation of hydrogen-rich carbon-containing products (Figure 5b, eqs. 15-17). In this work, more protons tend to recombine to form H2 on the Mo2C, so that less protons may participate in the CO2 reduction, which favors the generation of hydrogen-deficient carbon products and leads to the selective evolution of CO (Figure 5c). CONCLUSION In summary, an efficient and selective CO evolution from photocatalytic CO2 reduction in water is achieved on a NM-free catalyst fabricated by loading CdS nanoparticles (~5 nm) onto Mo2C nanowires. The CO evolution rate and selectivity on the CdS/Mo2C catalyst are 29.2 µmol h-1 and 98.3%, respectively, which are significantly higher than those obtained on the NMbased CdS/Pt photocatalyst (5.8 µmol h-1, 72.5%). The improved photocatalytic performance originates from the Mo2C component, which supports uniform dispersion of CdS nanoparticles and promotes separation of photogenerated electron-hole pairs; and more importantly, the Mo2C provides active sites for adsorption and photocatalytic reduction of CO2. The selective CO evolution is ascribed to the predominant H2 evolution (rate: 60.4 µmol h-1) during the photocatalytic process, which deprives the protons that participate in CO2 reduction and facilitates the formation of hydrogen-deficient carbon products. Based on the CdS/Mo2C catalyst, one may expect to realize industry-level syngas production under mild conditions if some practical engineering problems can be addressed, for example, continuous feed/conversion of raw materials and discharge of products, and recycling/regeneration of sacrificial agents, or building a sacrificial-agent-free system. In addition, the insights into the general photocatalytic carbon chemistry may help to realize high-performance catalysts with active CO2 capture and conversion and controllable production of carbon-based chemicals to benefit the industry.

ASSOCIATED CONTENT Supporting Information Experimental details, SEM image, Raman spectra, GC-MS spectra, UV-visible spectra, Photocatalytic activity. This material is available free of charge via the internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected] (Y.-X. Pan); [email protected] (S. Xin); [email protected] (J. B. Goodenough)

Author Contributions #

Y.-L. Men and Y. You contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge the funding support from the National Natural Science Foundation (21503062, U1662138, 21622606) and National Key R&D Program (2017FYA0205300, 2016YFA0202900) of China. The theoretical calculations and reaction mechanism study performed in Austin, TX, was supported by the Lawrence Berkeley National Laboratory BMR Program (Grant 7223523).

The authors thank Jo Wozniak from the Texas Advanced Computing Center for the art design.

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Artwork for TOC

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Selective CO Evolution from Photoreduction of CO2 on a Metal

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